1. Field of the Invention
The present invention is directed to a high impact strength film grade polymeric composition. More particularly, the present invention is directed to a polymeric composition which includes linear low density polyethylene (LLDPE) and an ethylene-vinyl acetate copolymer (EVA) containing at least about 28% by weight vinyl acetate (VA), based on the total weight of the ethylene-vinyl acetate copolymer.
2. Background of the Prior Art
The use of LLDPE, that is, a copolymer of ethylene and a higher .alpha.-olefin, as a polymeric film has significantly increased in recent years as a result of its acknowledged superior strength characteristics. Indeed, it is these properties that are critical which predicate the use of that film in that its strength properties, especially its impact strength, are critical to its use in such applications as a wrapping material, as a bag, as a container and the like. Polymeric films employed in such applications are subject to dropping, sudden impact and other challenges to their structural integrity.
Although LLDPE film has indeed found wide acceptance in view of its desirable strength properties, especially impact strength, over earlier polymeric films such as film formed of ethylene homopolymers, i.e. high density polyethylene and low density polyethylene, there is still a recognized need in this art for polymeric films, used in wrapping, bagging and the like, which possess even greater impact strength at comparative film thickness or equivalent impact strength at thinner gauges than those provided by LLDPE film.
Although the use of a polymeric composition comprising LLDPE and a second polymer to provide improved impact strength has not been expressly suggested in the prior art, it is known to combine LLDPE with other polymers for various and sundry purposes including the use of such a composition as a ply of a laminate film. For example, U.S. Pat. No. 4,856,656 to Sugimoto et al. discloses a multilayered polymeric film which comprises a low density polyethylene-based resin layer; a polyolefin-based resin expanded layer; and a high density polyethylene-based resin layer. Among the ethylene polymeric materials utilizable in the first recited low density polyethylene-based resin layer are low density polyethylene (LDPE), LLDPE and copolymers of ethylene and polar vinyl monomers of which an ethylene-vinyl acetate copolymer (EVA) is preferred. It is noted that the '656 patent suggests a mixture of EVA and not more than 50% by weight LDPE, LLDPE, ionomer or the like as preferable. It should be appreciated that the EVA contemplated for use in the low density polyethylene layer has a VA content of 5 to 35% by weight, preferably 7 to 30% by weight.
Although many references in the prior art disclose polymeric compositions which include LLDPE and EVA there are very few such polymeric compositions useful in film applications, especially compositions which comprise an EVA where the vinyl acetate (VA) content is sufficiently high to produce a film of excellent impact strength. For example, U.S. Pat. No. 4,877,827 and U.S. Pat. No. 4,997,880, both to Van Der Groep, describe a polymeric composition that comprises 30 to 70 parts by weight of an ethylenepropylene-diene rubber (EPDM), 30 to 70 parts by weight of an EVA with a VA content of 18% to 40% by weight; and 1 to 25 parts of one or more other polymers. The latter, third "other" polymer is recited to be any thermoplastic or elastomeric plastic other than EPDM and EVA. Although the preferred third polymer can be any one of six rubber polymers it can also be an olefinic polymer such as LDPE or LLDPE. The composition of the '827 and '880 patents is employed as an additive to other polymers or polymeric compositions to improve impact strength, as a compatibilizer between a polymer and an inorganic filler, as a miscibility promoter of immiscible polymers and other uses, which, like these other functions, are unrelated to or divorced from its use as a film.
The above discussion establishes the absence in the prior art of a polymeric composition providing improved impact strength even over that provided by LLDPE films. This discussion also emphasizes the need in the art for an improved polymeric composition that can be employed to enhance the impact strength of polymeric films.